The specific aims of the proposed investigation to study the human immune response to type III, group B Streptococcus would be: (1) to use purified capsular polysaccharide isolated from type III, group B Streptococcus to develop serologic tests (RIA, ELISA, etc.) for the detection of serum antibody against this antigen in several patient populations, (2) to analyze the age-related and disease-related serologic response to natural infection with type III strains of group B streptococci in asymptomatic carriers (pregnant and non-pregnant women, neonates, infants, children, adult men) and in young infants with invasive disease (sepsis, meningitis, bone and joint infection, etc.), (3) to collect seroepidemiologic data in oder to identify those babies at high risk for the development of seroius type III group B streptococcal disease and to determine prospectively the specific attack rates for early and late onset type disease among neonates whose mothers have high concentrations of serum antibody versus those with low concentrations, (4) to determine the potential feasibility of employing a large molecular weight polysaccharide isolated from type III, group B Streptococcus as an immunogen in adult women as a means of preventing symptomatic infection in their neonates and young infants.